1. Field of the Invention
The present invention relates to an optical information recording medium that allows information signals with high signal quality to be recorded or reproduced by irradiating a thin film formed on a substrate with a high energy optical beam such as a laser beam. The present invention also relates to a recording and reproducing method as well as a recording and reproducing apparatus.
2. Related Background Art
Research and development is being carried out actively about a recording medium that allows information signals to be recorded or reproduced by irradiating a thin film formed on a transparent substrate with a laser beam narrowed down to a minute spot. A known write-once recording medium is one in which, for example, a TeOx (0<x<2) recording thin film is formed, on a substrate, of a mixture of Te and TeO2, which is a material containing, for instance, a metal element dispersed in an oxide that serves as a base material (see, for example, JP 50(1975)-46317 A). A large reflectance change can be obtained from this recording medium through irradiation with an optical beam for reproduction.
The TeOx recording thin film allows a crystalline recording mark to be formed by being irradiated with a laser beam in the amorphous state after being formed without being subjected to an initialization process, such as laser annealing. This is an irreversible process and does not allow the overwrite correction or erasure to be performed. Therefore the medium including this recording thin film can be used as a recording medium capable only of writing once. In the TeOx recording thin film, it takes some time until a signal is saturated after being recorded, i.e. until crystallization in the recording thin film caused by laser beam irradiation proceeds sufficiently. Accordingly, unless the medium including this recording thin film is improved, it is not suitable as a medium that requires high-speed response as in the case of, for example, a computer data file in which data is recorded on a disk and is verified after one rotation. In order to accommodate this disadvantage, it has been proposed to add, for example, Pd or Au as a third element to TeOx (for example, see JP 60(1985)-203490 A, JP 61(1986)-68296 A, and JP 62(1987)-88152 A). Pd and Au are considered to have the function of promoting crystal growth of Te in a TeOx recording thin film at the time of laser beam irradiation. Therefore crystal grains of Te and a Te—Pd alloy or a Te—Au alloy grow at high speed. Pd and Au have a high oxidation resistance and do not impair the high moisture resistance of the TeOx recording thin film.
Generally, the following method is used as a basic measure for increasing the amount of information that one medium can handle. That is, the wavelength of the laser beam is shortened or the numerical aperture of an objective lens for focusing a laser beam is increased, so that the spot diameter of the laser beam is reduced to improve the areal recording density. Furthermore, recently, a medium with a multilayer structure including a plurality of information layers stacked together also has been put into practical use. In order to achieve such high-density recording and multilayer recording, a recording medium also has been proposed that is obtained by improving the film thickness and composition of a recording material containing TeOx and, for example, Pd or Au added thereto (see, for example, WO 98/09823 (pp 20-23, FIG. 4)). Moreover, the recording density is increased through an optical enhancement effect and an effect of suppressing the thermal interference between marks by cooling (see, for example, JP 2002-251778 A). These effects are obtained by addition of a reflective layer.
When the high density recording as described above is performed, especially recording using a blue-violet laser, a recording layer (recording thin film) may not be suitable for high density recording. This is because a recording layer may be damaged due to the heat load imposed by laser heating in some cases, which may cause, for instance, an increase in noise, and thereby the quality of a recording signal may deteriorate. In order to prevent this, it is effective to provide a protective layer produced with a material such as a dielectric. Examples of the properties required of the protective layer include (1) a high heat resistance and protecting a recording layer from thermal damage, (2) a high adhesiveness with an adjoining material such as a recording layer and undergoing, for example, no separation, corrosion, and diffusion even under conditions of a high temperature and humidity, (3) a high transparency and a suitable refractive index, and enhancing the optical change of a recording layer, and (4) being thermally stable and preventing the grain size and composition distribution from changing even at a high temperature and humidity. Particularly in a write-once recording medium, although it is important for it to allow high density recording to be performed, it is very important for it to have a high storage reliability. Even if sufficiently high signal quality was obtained at the time of recording, the effect of thermal damage that was not manifest immediately after recording may become evident later on and causes an increase in noise when the recording medium is left, for example, under a high temperature or humidity environment.
In order to prevent the quality of a recording signal from deteriorating due to, for example, an increase in noise, which results from the damage of a recording layer caused due to the heat load imposed by laser heating as described above, it also is effective to provide a layer with a high radiation performance, such as metal, as a means other than providing the protective layer produced with a material such as a dielectric. Generally, the above-mentioned layer is formed using a material with a suitable optical constant to be allowed to have a reflection function, that is, the layer is formed to serve as a reflective layer, and thereby the optical interference effect is used to increase the optical absorptance of a recording layer to improve the recording sensitivity and further to enhance the optical change of the recording layer. The reflective layer also is required to have high heat resistance, adhesiveness with an adjoining layer, and further high reliability under conditions of high temperature and humidity.